IB/mad: Verify mgmt class in received MADs

If a received MAD contains an invalid or reserved mgmt class, we will
attempt to access method_table outside of its range.  Add a check to
ensure that mgmt class falls within the handled range.

Found by code inspection.
Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/core/mad.c

@@ -1596,6 +1596,9 @@ find_mad_agent(struct ib_mad_port_private *port_priv,
 					mad->mad_hdr.class_version].class;
 			if (!class)
 				goto out;
+			if (convert_mgmt_class(mad->mad_hdr.mgmt_class) >=
+			    IB_MGMT_MAX_METHODS)
+				goto out;
 			method = class->method_table[convert_mgmt_class(
 							mad->mad_hdr.mgmt_class)];
 			if (method)